Current mobile phones are designed to operate in multiple radio frequency bands to be compatible with the existing telecommunication standards (GSM, 3G, WCDMA, LTE), which may vary from country to country. The 3G standard, in one country, requires five pairs of bands. Two bands of a pair are respectively used for transmission and reception in full duplex. If the LTE standard is added, seven pairs of bands are to be in the same country. If it is desirable to cover the bands used for the 3G and LTE standards in all countries, ten pairs of bands are to be used.
FIG. 1 is a schematic diagram of a transceiver stage of a mobile phone designed to handle one pair of bands. The transceiver stage includes a duplexer 10 formed by a pair of bandpass filters, one for conveying an incoming signal Rx arriving on an antenna, the other for transmitting on the antenna an outgoing signal Tx arriving via a power amplifier PA.
FIG. 2 is a diagram illustrating the response curves, one in a solid line and the other in a dashed line, of the two bandpass filters of a duplexer. The frequency response of each filter is characterized by the bandwidth B, the gap between the two bands BG, the insertion loss IL (attenuation compared to the level of the original signal), the rejection ratio R from the other band, and the slope S between the bands.
The characteristics that may be of a more particular interest are the slope S and the rejection ratio R, which increase when both bands become closer. A worst case, for example, is for the LTE US and LTE EU standards, which require a 20 MHz gap BG between bands at frequencies around 2 GHz. For example, for the LTE US standard, the transmission band is between 1850 and 1910 MHz, and the reception band is between 1930 and 1990 MHz.
Thus, templates may be imposed on the duplexer filters. These templates have typically been met in practice by surface acoustic wave filters (SAW) or bulk acoustic wave filters (BAW).
SAW or BAW filters, having a very limited frequency range for adjusting their bandwidth, cannot typically be adjusted to cover multiple bands. The result is that a separate duplexer is used for each pair of bands, and that ten duplexers may be required with corresponding auxiliary circuits (amplifiers PA), independently switched on the antenna, if a universal mobile phone platform is desired.
In theory, active filters or passive elliptical filters may satisfy the templates. Such filters may have a satisfactory frequency adjustment range, which may reduce the number of duplexers by making each duplexer tunable over several neighboring bands. However, the power consumption of active filters may be prohibitive for battery-operated devices. As for elliptical filters, there are no inductors on the market with a sufficient quality factor, so that the actual characteristics of these filters differ too much from the theoretical characteristics that satisfy the templates.